Today, renewable energy sources including solar cells are attracting attention as alternative energy, in that they do not use fossil fuels that restrictively exist in the Earth and can minimize environmental pollution. However, the renewable energy sources have voltages and currents that are unstable, and thus it is difficult to supply a stable power to loads. Accordingly, a technique has currently been being developed to stabilize power output of a system by using an energy storage device such as a battery.
A conventional renewable electric generator system has a structure in which a power is supplied from a DC battery power source to loads by connecting a power converter thereto.
Typically, ripples are generated in a DC voltage when power conversion is performed by stepping up or down the DC battery power to a desired voltage level. Conventionally, a high-capacity electrolytic capacitor was used to eliminate the ripples of the DC voltage. However, the electrolytic capacitor has a high capacity and short lifespan.
FIG. 1 illustrates an example of an electrolytic-less capacitor type ripple-eliminating circuit of a power converter according to a related art. FIG. 1(a) illustrates a configuration in which a ripple-eliminating circuit (relating to a ripple port) is connected in parallel (shunt) with a power-converting circuit, and FIG. 1(b) illustrates a configuration in which a ripple-eliminating circuit is expanded from two ports to three ports to be connected by adding windings in a transformer-type ripple-eliminating circuit.
The reason that an electrolytic-less capacitor type ripple-eliminating circuit is used will be described as follows. In order to output an AC output, i.e., an AC current, the AC current passes through an intermediate portion of the circuit, and thus a voltage of a capacitor positioned in the center significantly fluctuates. A high-capacity electrolytic capacitor is conventionally employed in a ripple-eliminating circuit to suppress such fluctuation. However, using the high-capacity electrolytic capacitor causes an increased price, reduced reliability, and a very high boosting ratio, thereby deteriorating power-converting efficiency.
A background technique of the present invention has been disclosed in Korea Patent Laid-Open Publication No. 2006-0094466 (Aug. 28, 2006)